More specifically, the present invention relates to bypass (dual flow) turbojet engines comprising:                a nacelle delimiting, at the front, an air intake and axially containing a cold flow fan and a hot flow generator provided with a turbine enclosed in a casing ending, at the rear, in a bulbous boss connected to said turbine casing along a first connecting surface;        an engine cowl containing said hot flow generator and comprising:                    an external wall delimiting with said nacelle a cold flow duct ending, at the rear, in a cold flow nozzle, and            an internal wall delimiting with said turbine casing a hot flow duct,                        a hot flow nozzle extending said engine cowl rearward by being connected thereto along a second connecting surface and collaborating with said boss to delimit a nominal nozzle throat section and a nominal outlet section for said hot flow, and        at least one ventilation opening positioned between said external wall of said engine cowl and said hot flow nozzle, on the outside with respect to said cold flow nozzle, and intended to discharge to the outside a ventilation air flow bled from said cold flow and introduced into said engine cowl in order to regulate the temperature of said hot flow generator.        
In turbojet engines of this type, said hot flow nozzle is therefore subjected to the cold flow on the external side and to the hot flow on the internal side, and comprises an external skin that is optimized for the flow of the cold flow and an internal skin that is optimized for the flow of the hot flow.
A double-skinned structure such as this is therefore relatively heavy and, in addition, because of the substantial temperature difference between the hot flow and the cold flow, behaves like a bimetallic strip and undergoes deformations that detract from the optimization of the flows of the hot and cold flows in flight, particularly in cruising flight.
It is an object of the present invention to overcome these disadvantages.
To this end, according to the invention, a turbojet engine of the type recalled hereinabove is noteworthy in that:                said hot flow nozzle consists of a single skin;        said single skin is shaped to encourage said cold flow to flow when the aircraft is in cruising flight; and        the shape of said bulbous boss is tailored in order to be able to respect said nominal nozzle throat section and said nominal outlet section for said hot flow.        
The present invention is based on the following observations:                that for most of the time that an aircraft turbojet engine is in use it is in cruising flight and that it is therefore advantageous to optimize the rear part of such a turbojet engine for cruising flight;        that in cruising flight, the flow of the cold flow of such a turbojet engine is a supersonic flow, whereas the flow of the hot flow is a subsonic flow; and        that optimizing the flow of the supersonic cold flow is more critical than optimizing the flow of the subsonic hot flow, and that optimizing the flow of said cold flow can be taken almost exclusively into consideration provided that the nominal nozzle throat section and nominal outlet section are respected for said hot flow.        
Thus, according to the present invention, these observations are put to good use to create the hot flow nozzle in the form of a single skin, something which, by comparison with a double-skin embodiment, provides a weight saving and avoids bimetallic strip effect deformations.
In order to avoid excessive disruption to the hot flow, it is also advantageous to ensure that the shape of said single skin avoids any separation of said hot flow.
Advantageously, along the axis of the turbojet engine, the shape of said single skin varies continuously. Said single skin is bulbous in shape, for example, widening from said second connecting surface into the continuation of said external wall of said engine cowl.